An MRI apparatus arranges a test object in a uniform static magnetic field space, applies a high-frequency magnetic field and a gradient magnetic field to the test object according to a predetermined pulse sequence, and causes a nuclear spin in a certain cross section of the test object to magnetically resonate. Then, the MRI apparatus detects the resulted nuclear magnetic resonance signal, reconstructs the detected signal into an image using two- or three-dimensional Fourier transform or the like, and displays tomographic images.
MRI apparatuses are classified into horizontal magnetic field apparatuses and vertical magnetic field apparatuses depending on the direction of the static magnetic field. In any of the magnetic field apparatuses, a magnetic resonance signal is detected using an RF receiving coil disposed in proximity to the test object. The RF receiving coil needs to be disposed in the direction in which the magnetic resonance signal (magnetic field) perpendicular to the direction of the static magnetic field is detected. Accordingly, the configuration of the receiving coil varies depending on the direction of the static magnetic field.
In a horizontal magnetic field apparatus, by using a tunnel-shaped magnet, a static magnetic field is generated in the same direction as the center axis of the tunnel, and a test object is positioned in the static magnetic field such that the body axis of the test object is along the direction of the static magnetic field. Accordingly, the RF receiving coil needs to be disposed in the direction in which a magnetic field perpendicular to the body axis of the test object is detected, so a saddle-shaped coil or a loop coil to be disposed on the surface of the test object is often used for the RF receiving coil.
On the other hand, in a vertical magnetic field apparatus, two magnets are disposed in the vertical direction, a static magnetic field is generated in the vertical direction between the magnets, and a test object is positioned in the static magnetic field such that the body axis of the test object is perpendicular to the direction of the static magnetic field. Accordingly, the RF receiving coil can be disposed along the direction in which a magnetic field in the body axis direction of the test object is detected, so, conventionally, a solenoid coil wound around the circumference of the test object was often used. Also, a technique of using together two coils the magnetic field directions of which are perpendicular to each other to perform QD (Quadrature Detection) combining in order to improve the sensitivity. In the vertical magnetic field apparatus, in addition to the solenoid coil for detecting the magnetic field in the body axis direction, a saddle-shaped coil for detecting a magnetic field in the body width direction can be used together to perform QD combining.
The sensitivity and SN ratio of the RF receiving coil of the MRI apparatus improve more as the distance to the test object decreases more. Accordingly, the shape of the receiving coil is desirably determined so as to follow the shape of the test object depending on the size and shape of the test object so that an air gap between the test object and the RF receiving coil pattern is as small as possible. Conventionally, various RF receiving coils for accommodating the directions of static magnetic fields and the sizes and shapes of test objects have been invented. However, many of them are fabricated by winding a coil pattern around a bobbin that is made of a resin and is poor in flexibility.
In order to improve this problem, Patent Document 1 describes that both flexible and rigid, nonconductive support members for supporting an RF receiving coil pattern are used, in which flexible parts and rigid parts are alternately connected to each other in the circumference direction into a cylindrical shape. This facilitates the RF receiving coil to be wound around and closely fitted to the test object.
Also, the cylindrical-shaped RF receiving coil described in Patent Document 1 includes connector sections for connecting and disconnecting the coil pattern. When the connector sections are connected, two coil systems (solenoid coil and saddle-shaped coil) that have magnetic field components perpendicular to the static magnetic field direction and perpendicular to each other can be formed. The intersection part of the two coil systems is formed as rigid part to limit the flexibility of deformation, and the coil members of the intersection part are separated from each other by the distance of at least 5 mm. This separation prevents the two coil systems from being electromagnetically coupled.
Also, by determining the lengths of the flexible parts and rigid parts appropriately depending on the area of the test object is to be imaged, the RF receiving coil having a good reception sensitivity and a wide uniform reception area was achieved.